1. Field of the Invention
The invention relates to variable gain distributed amplifiers and more particularly to variable gain distributed amplifiers implemented in the Monolithic Microwave Integrated Circuit (MMIC) format and suitable to digital control.
2. Prior Art
Variable gain distributed amplifiers are employed when broad band signal processing (i.e. varying gain or attenuation) at microwave frequencies is desired. A known technique is to interconnect a plurality of solid state gain elements, typically field effect transistors, with serial transmission line elements interconnecting the signal input electrodes and signal output electrodes. The serial transmission line elements are then designed to provide series inductive reactances to compensate for shunt capacitive reactances at the input (i.e. signal gate) electrodes and at the output (i.e. drain) electrodes, thereby creating a multisection low pass transmission line. Artificially, the serial reactances can be selected to create relatively high cut-off frequencies compatible with microwave signals of high relative bandwidth.
In variable gain distributed amplifiers a recurrent problem is not only the linearity of the phase and gain response with frequency, but also constancy in the phase response from gain setting to gain setting. Achieving accuracy in phase and amplitude response is more difficult to achieve when wide band operation is sought.
The individual field effect transistors in a distributed amplifier may be controlled for different gain settings. One known distributed amplifier utilizes dual gate field effect transistors in which the signal is applied to the first or signal gate and an analogue gain control voltage is applied to the second or control gate.
The problem with this mode of control of the gain of individual field effect transistors is that the parasitic reactance of the device varies with the voltage on the control gate. While some tailoring may be used to improve the tracking between devices and to achieve linearity in response at one gain setting, the phase response from gain setting to gain setting due to this change in parasitizes differs very substantially.
A recurrent problem in distributed amplifier design is that of avoiding instability at the margins of the operating bandwidth. One known solution is to provide roll-off in individual amplifier sections. This solution undesirably reduces the linear bandwidth of the distributed amplifier.
A known device in which gain may be digitally controlled is a segmented dual gate field effect transistor. In these transistors the transconductance (G.sub.m) may be scaled in a desired manner (i.e. binary, geometric, logarithmic) by selective activation of suitably scaled individual segments. When the individual segments are operated at high gain, on state settings, the transconductance (G.sub.m) becomes proportional to the gate width of the segment. The parasitizes, i.e. the signal gate to source capacity and the drain to source capacity, however, remain that of the full device, and remain substantially constant. Such devices are described in U.S. Pat. No. 4,638,190 and 4,734,751 to Y. C. Wang et al and assigned to the Assignee of the present application. Such devices are characteristicly fabricated in the MMIC format, while active and passive components are formed on the monolithic substrate.